Liver tissue contains factors which inhibit the proliferation of liver cells. These hepatoantimitotic factors, often referred to as hepatic chalones, are isolated in fractions containing polypeptides and proteins of 1,000 to 40,000 daltons. Purification of these factors has been hampered by their hydrophobic properties. We have developed methods for the fractionation and purification of hydrophobic peptides and proteins by high-pressure liquid chromatography on silica gel which may be of special value in shalone isolations. We propose to use these procedures, in combination with classical fractionation methods, to isolate hepatoantimitotic molecules and determine their structural characteristics. Biological activities of these factors will be assayed by their effect on the proliferation of cultured hepatoma cells and normal liver tissue and in vivo on regenerating rat liver. These studies may lead to the development of new antineoplastic drugs for the control of hepatoma growth and clarify the role and/or validity of the postulated hepatic chalone.